There are numerous reports of people losing internet access on their mobile phones

BBC Persian correspondent Kasra Naji said a common factor in all locations has been protesters' demand for an end to clerical rule in Iran.

There is also anger at Iran's interventions abroad. In Mashhad, some chanted "not Gaza, not Lebanon, my life for Iran", a reference to what protesters say is the administration's focus on foreign rather than domestic issues.

Iran is a key provider of military support to the government of Bashar al-Assad in Syria. It is also accused of providing arms to Houthi rebels fighting a Saudi-led coalition in Yemen, which it denies, and is an ally of Lebanon's powerful Shia movement Hezbollah.

BBC Persian, which broadcasts on TV, on radio and online from London, is banned in Iran – where staff and their families routinely face harassment and questioning from the authorities.

What has been the response, at home and abroad?

The Iranian authorities are blaming anti-revolutionaries and agents of foreign powers for the outbreak.

Also on Saturday, thousands of pro-government demonstrators turned out for pre-arranged rallies to mark the eighth anniversary of the suppression of the 2009 street protests.

The US has led international support for the protesters.

President Donald Trump tweeted: "Oppressive regimes cannot endure forever, and the day will come when the Iranian people will face a choice. The world is watching!"

Iran's foreign ministry called earlier comments from Mr Trump and other US officials "opportunistic and deceitful".

Who are the Revolutionary Guards?

Iran's Islamic Revolution Guards Corps (IRGC) was set up shortly after the 1979 Iranian revolution to defend the country's Islamic system.

It has since become a major force in the political, economic and military life of the country, with its own ground forces, navy and air force. It controls a volunteer militia of tens of thousands of people – the Basij Resistance Force.

The IRGC sometimes works alongside police, and it was announced in December that it would do this again.

Its stature says it will co-operate with law enforcement forces "when necessary" but there has been some controversy over its scope when dealing with civilians.